Intraspecific Population Regulation
Chapter 11
Population growth
In environments where resources are limiting, growth is constrained
As density increases, intraspecific interactions tend to regulate the size of the population
Physiological, morphological and behavioral adaptations
Exponential growth model
Assumes:
Essential resources are unlimited
Environment is constant
Mortality and fecundity are not affected by resources/density
In limited environments
Birth and death rates do change with density
Add changing rates to exponential model
Exponential model
dN/dt = (b-d) N
Model under limiting resources
dN/dt = [(b0 – aN) – (d0 + cN)]N
As N increases, birth rates decline and death rates increase, which slows population growth
Carrying capacity
dN/dt = [(b0 – aN) – (d0 + cN)]N
Set dN/dt equal to zero and solve to find:
K = (b0 - d0)/(a+c)
Logistic growth model
dN/dt = rN (1-N/K)
r = b0 - d0
Population size versus time
Growth rate versus population size
Compare to exponential model
Squirrel life table data
r = 0.18 (from life table)
K = 200 (arbitrarily set)
Density dependence
Density dependent effects influence a population in proportion to its size
Logistic model as presented assumes that birth rates and death rates change with density (directly - resources, indirectly - disease)
Density dependent regulation
Allee affect
Extinction rather than equilibrium when N < A
Competition
Resource is in short supply and affects survival, reproduction and/or growth
Scramble competition
Growth and reproduction are depressed equally
Can lead to local extinction
Contest competition
Some individuals win
Competition
Exploitation
Indirect
Interference
Direct
Reduced growth with density
Differential allocation with density
Survival effects
Self-thinning
Density declines
Average biomass increases
Density affects fecundity
Density affects reproduction in plants
Stress under high density
Stress reduces reproduction
Supresses immune system
May be mediated through pheromones
Dispersal
Subadults usually driven out by adult agression
Probably does not function as population regulation
Population expansion & enhance outbreeding
Social behavior
Home range
Body size is related to home range
Territory is a defended area
Floaters hold no territory
Philopatry
Mechanism of population regulation when there are an excess of individuals
Territoriality
Removed individuals replaced by floaters
Plant “territoriality”
Density independent
Proportion affected is same at any density
Adverse weather conditions
Density independent regulation
Density-dependent and –independence act together